I had a youtube viewer make the comment that the Nikon D600 and Canon 6D are the same in higher ISOs when you "resize" the D600's pixel dimensions to that of the Canon 6D. My results as demonstrated were to the contrary.
Up to 3200-6400 (depending on the subject matter) they are very similar. 12800 and higher? The Canon 6D is much cleaner, particularly when it comes to rendering blacks. The D600's shadows start taking on a purplish hue. Not only is this visible in full size jpegs, its visible in resized images, as well as video which was clearly demonstrated on my Nikon D600 vs Canon 6D Epic Shootout. To it's benefit, the D600 has better dynamic range.